An example of a component having a thin-walled cup portion is a universal joint yoke for use in a propeller shaft of a power train system. FIG. 12 schematically shows a propeller shaft. The propeller shaft connects the outlet of a transmission (111) and a final drive (119) in a power transmission system and uses universal joints at connections of shafts thereof. The universal joints each include a yoke (112a, 112b or 112c) and a cross member (113a, 113b or 113c) and are adapted to transmit rotation as power to shafts (114a and 114b) connected thereto. Denoted by reference numeral 116 is a metal fitting that mounts the propeller shaft on a body 117, and by numeral 115 is a bearing for angle change.
Universal joint yokes have been manufactured from an extruded aluminum alloy by means of cold forging. FIG. 13 shows an example of a die configuration for use in a conventional cold forging process. FIG. 16 exemplifies an as-forged article with flash that is obtained by a conventional flash-emerging forging process (FIG. 16(a)), and a yoke that is obtained by trimming the flash (FIG. 16(b)).
In FIG. 13, an upper die 2 supported on an upper bolster 1 gives an aluminum workpiece a shape corresponding to a cup portion of a universal joint yoke to be connected to a shaft, and a lower die 4 on a lower bolster 5 gives the aluminum workpiece a shape corresponding to a pin boss portion to be coupled to a cross member. The upper die 2 descends toward the lower die 4 to press an aluminum workpiece 6 therebetween, whereby the shapes of the upper and lower dies are transferred to the workpiece by means of a space that is defined by the upper and lower dies. The aluminum workpiece placed in the lower die is lubricated beforehand and is not preheated. FIG. 14 shows the die configuration for use in the conventional cold forging as viewed at the forging completion stage where the upper die is positioned at the bottom dead center of its stroke. A cup portion of an as-forged article 7 formed on the side toward the upper die is formed through free-end forging not involving restraint by the upper die. Reference numeral 3 denotes a stopper.
As shown in FIG. 16(a), the as-forged article 7 obtained by the conventional process involves an uneven height of an excess metal end for the following reason. Since a cup portion 41 is formed through free-end forging, workpiece material undergoes plastic flow such that much more workpiece material flows toward a pin boss portion 43, so that workpiece material that flows toward the cup portion 41, which is located axially opposite the pin boss portion 43, is diminished. As a result, the quantity of cutting increases, resulting in a decrease in yield in terms of material utilization.
As shown in FIG. 14, in the conventional forging process, forging starts upon the upper die contacting the workpiece, and the outer circumferential portion of the upper die and the inner circumferential portion of the lower die do not come into contact during the course of forging. A pin boss portion is irregularly shaped because of the difference in amount of the material flowing into the cavity of the lower die. This irregular shape of the pin boss portion causes deflection of the upper die during the course of forging. As a result, concentricity becomes about 1 mm between the inner cylindrical wall of a cup portion, which is formed by means of the upper die, and the outer cylindrical wall of the cup portion, which is formed by means of the lower die. Therefore, in the subsequent machining step, the wall thickness of the cup portion is finished uneven. In some cases, the resultant yoke may exhibit insufficient coupling strength in coupling to a shaft.
In the conventional forging process, since the occurrence of underfill on a cup portion is unavoidable, a large wall thickness is imparted to the cup portion. Also, in order to obtain a cutting allowance, excess metal is intentionally added even though the height of an excess metal end becomes uneven. Therefore, poor yield in terms of material utilization results. Also, a height variation of a cup end of an as-forged article is not less than 25 mm, which is in excess of a preferred value of 10 mm.
In order to reduce a cutting allowance through attainment of a uniform height of a cup portion, a closed forging process has been devised. However, in some cases, the closed forging process has involved the following problem. Since the volume of a pin boss portion accounts for a great portion of the entire yoke volume in the course of plastic flow of workpiece material toward a pin-boss-forming cavity, plastic flow of workpiece material is not initiated toward a portion of a cup-forming cavity that is located axially opposite the pin-boss-forming cavity. Therefore, in some cases, there has arisen a problem of underfill in a corresponding region of a cup portion, which is a coupling portion to a shaft. Also, since plastic flow of workpiece material to a portion of the cup-forming cavity that does not have a counter pin-boss-forming cavity occurs preferentially, filling of the portion of the cup-forming cavity is completed at an early stage. Thus, a die load increases, thereby shortening the life of a portion of the die that corresponds to the portion of the cup-forming cavity of early completion of filling. Therefore, the conventional closed forging process is not practical.
In order to solve the above-mentioned problem, multi-stage forging was proposed. Specifically, first a pin boss is formed, and then a cup is formed. FIG. 15 shows a die for use in a conventional multi-stage forging process. An upper die 31 supported on a bolster 1 and a lower die 32 disposed on a bolster 5 shown in FIG. 15(a) are used to forge a perform 34 with a pin boss portion. The perform is then forged into a perform with both the pin boss portion and a cup portion using an upper die 2 and a lower die 4 shown in FIG. 15(b). Reference numeral 33 denotes a knockout pin, and numeral 3 a stopper. This process requires dies for use at individual stages and involves a plurality of forging stages, resulting in low productivity and high cost.
Such multi-stage forging requires a plurality of dies and thus involves high die cost. Further, since plural stages are involved, productivity decreases. Therefore, multi-stage forging is not practical. Also, as mentioned previously, single-stage forging involves a heavy forging load and thus requires a forging press of a large forging capacity. Further, an increase in die load shortens the life of a die. Thus, demand exists for a single-stage forging process with enhanced productivity.
JP-A No. 2000-263178 discloses a method for forming a uniform-height cup-shaped article of brass. According to this method, first a workpiece is formed into a cup shape, and subsequently a separately driven ram presses the cup-shaped workpiece from above to form an undercut on the workpiece. The method disclosed in the publication uses a workpiece of brass. Since brass is readily susceptible to plastic flow, a workpiece of brass can be formed under the disclosed conditions. However, because of high resistance to deformation at high temperature, an aluminum alloy is less susceptible to plastic flow as compared with brass and thus cannot be formed under the disclosed conditions.
The present invention has been achieved in view of the foregoing, and an object of the invention is to provide a method for manufacturing a universal joint yoke which exhibits good yield in terms of material utilization through obtainment of a universal joint yoke preform whose shape is close to that of a finished product, whose excess metal to be trimmed is diminished, and whose cup portion is of uniform height.